An image sensor is a semiconductor device converting an optical image to an electrical signal. Types of image sensors include CCD (Charged Coupled Device) image sensors and CMOS (Complementary Metal Oxide Semiconductor) image sensors.
A CMOS image sensor is a device converting an optical image into an electrical signal using CMOS manufacturing technology. The CMOS sensor adopts a switching method which makes MOS transistors of the number of pixels and uses them to detect outputs in sequence. A CMOS image sensor is, compared to a CCD image sensor, easier to operate, flexible in scanning methods, and scales down final products by integrating image sensing and signal processing circuits into a single chip. Further, since the CMOS image sensor is manufactured using a generally compatible CMOS technology, manufacturing prices can be lowered, and energy consumption of the CMOS image sensor is greatly decreased.
In a manufacturing process of an image sensor, efforts are made to maximize photo sensitivity. One approach is to optimize a light condensing apparatus. For example, a CMOS image sensor includes a light detector which detects light, and a logic circuit which processes detected light to data through an electrical signal. A photodiode is used as a light detector. When manufacturing a CMOS image detector with this configuration, the area of the light detectors over the entire image sensor needs to be increased to increase photo sensitivity. However, an area used for the logic circuit limits the area of light detector because the light detector can only be formed where the logic circuit is excluded. Thus, light condensing technologies which alter the path of light being projected onto areas other than the light detector have been studied. One of the light condensing technologies is a micro lens over a color filter of the image sensor.
An image sensor with such a micro lens and manufacturing method thereof in accordance with the related art is explained by referring to FIGS. 1A to 1E, which are cross-sectional views of a semiconductor device. As illustrated in FIG. 1A, for example, a silicon nitride film-based protective layer 21 is formed over a semiconductor substrate 10, which has a light detector 13 including photodiode 11 and a circuit bonding pad. The circuit bonding pad is exposed by a photolithography process for removing the protective layer formed over the circuit bonding pad. Here, the photolithography process is performed by: coating photoresist and pattering it; removing parts of the protective layer 21 by etching; and removing remaining photoresist by reactive ion etching.
Thereafter, as illustrated in FIG. 1B, a color filter array 23 is formed over the protective layer 21. Here, the color filter array 23 is a combination of a red filter R, green filter G, and blue filter B formed by coating, exposing, and developing photoresist which includes pigments of specific colors such as red, green, and blue.
Then, as illustrated in FIG. 1C, a planarization layer 25 is formed over the color filter array 23. The planarization layer 23 is for stepped portion restoration of the color filter layer 23, uniform manufacturing and focal distance control of the micro lenses 27. Here, the planarization layer 25 can be formed with an insulating film such as photoresist, oxide film, or nitride film.
Next as illustrated in FIG. 1D, the photoresist is sequentially coated, exposed, and developed over a surface of the planarization layer 25. Then, micro lenses 27 are formed by bleaching the photoresist, heat-processing to reflow the photoresist to shape lenses, and hardening.
Thereafter, as illustrated in FIG. 1E, a protective layer 29 is formed over the micro lenses 27. Since the photoresist used as the micro lenses 27 is a weak solid, the micro lenses 27 can be broken when particles generated from wafer sawing are stuck on the surface, and therefore, the protective layer 29 is used. Here, the protective layer 29 is a deposition of USG (Un-doped Silicate Glass) at a low temperature (at about 180° C.), and this is called LTO (Low Temperature Oxide).
As illustrated above, in the related art, the image sensor obtained by an image sensor manufacturing method has gaps r to prevent bridge phenomenon between adjacent micro lenses 27. The gaps r decrease fill factor of micro lenses 27 and light incident through those gaps cause crosstalk. These problems are increasingly significant as devices are miniaturized.